In the past, empirical calculations were used to control gas nitriding and nitrocarburizing processes sans any atmosphere process control. Generally, the nitriding process was controlled by adjusting the flow rate of ammonia gas, carbon dioxide, endothermic gas and nitrogen; sometimes an Orsat device was used to determine the amount of ammonia exhausted from the furnace.
This process mostly led to reproducible nitriding results, particularly in processes utilizing constant atmosphere flow rates. However, in pure nitriding and nitrocarburizing procedures, deviations occasionally occur in the resulting part microstructure for unspecified reasons.
To overcome this issue, a reliable and continuous process control system was developed that can monitor deviations in the furnace’s gas atmosphere. In nitriding and nitrocarburizing processes, atomic nitrogen is formed through a catalytic decomposition of ammonia on the metal surface of a work piece.
This decomposition conform to the following reaction equation: NH3-> [N] + 3/2 H2. The decomposition of ammonia also occurs in the furnace atmosphere, but here instead of atomic nitrogen, molecular nitrogen is formed.
Depending on the quantity of atomic nitrogen that is available to a part, different iron nitrides will form in the white layer. But, since the concentration of atomic nitrogen can be utilized to measure the composition of the white layer, the nitriding potential (Kn) or nitriding activity of the furnace atmosphere can also be employed for this purpose.
When the nitriding potential is higher, then the nitriding effect also tends to be higher and more nitrogen will be available to the parts. Kn refers to the quotient of the partial pressure of the ammonia, which still exist in the furnace and H2 refers to the partial pressure of hydrogen that has already formed through ammonia dissociation.
When using ammonia and nitrogen during a nitriding reaction, ammonia becomes the only source of hydrogen. Therefore, the hydrogen content which is measured can be quantified from the amount of decomposed ammonia.
The amount of ammonia available for nitriding can be measured by deducting the amount of ammonia gas in the furnace from the amount of incoming ammonia gas. This gives information about the present composition of the gas in the furnace, i.e. the molecular nitrogen and hydrogen formed by decomposition of ammonia and the ammonia left in the furnace is still available for the nitriding reaction.
In case the incoming gas contains hydrogen and ammonia, then the amount of incoming hydrogen should also be subtracted from the hydrogen calculated by the HydroNit sensor before quantifying the amount of ammonia still available for the nitriding reaction.
Moreover, the amount of nitrogen flowing into the furnace should also be considered when measuring the partial pressures of particular gases in the furnace. Equation Kn=p(NH3)/p (H2)3/2 can now be utilized to measure the Kn from the estimated partial pressures for ammonia and hydrogen.
Figure 1. HydroNit sensor.
When the HydroNit sensor (Figure 1) is used, the furnace atmosphere is calculated directly. However, in other sensors the furnace atmosphere is forced out of the furnace via pies or hoses prior to reaching the sensor.
Conversely, the thermochemical and chemical reactions that take place in the gas as it passes from the furnace to the sensor can impact the calculated value and might cancel the measured Kn.
Figure 2. The HydroNit sensor consists of a protective shield that is inserted into the retort.
A protective shield in the sensor is inserted inside the retort, as shown in figure 2. This shield includes a measuring tube made from a material, which is permeable only to hydrogen. Hence, the result is not impacted by cross-sensitivities from other parts of the furnace atmosphere.
During a nitriding reaction, the process of ammonia dissociation induces a variation in partial pressure and hydrogen concentration between the interior of the measuring tube and the furnace atmosphere. This variation permits the hydrogen to diffuse via the wall and into the measuring tube. This diffusion continues until the pressures and concentrations are same. In other words, the pressure calculated by the sensor is same as the hydrogen partial pressure in the furnace atmosphere.
After the measurements are over, Kn is measured and can be utilized to regulate the heat treatment process. If the measured Kn is less than the required value, then nitriding will not take place as anticipated. In such cases, the flow rate of hydrogen is reduced until the measured Kn is equal to the desired value. However, in case reduction of hydrogen flow rate does not raise the measured potential to go with the desired potential, then the flow rate of ammonia can be increased.
If the measured potential is higher than desired, then the flow rate of ammonia can be reduced until the desired value is equal to the exact value. On the other hand, if the ammonia is reduced to the lowest flow rate needed to perform the process and the nitriding potential is still higher than required, then the flow rate of hydrogen can be raised. Figure 3 shows a schematic circuit for this kind of control system.
Figure 3. Schematic of nitriding potential control with flow control.
HydroNit sensors and computer control systems are therefore important for the nitriding process and improve the reliability and repeatability of the nitriding and nitrocarburizing processes.
This information has been sourced, reviewed and adapted from materials provided by Ipsen.
For more information on this source, please visit Ipsen.